This invention relates to devices and methods for transforming an optical wavefront in accordance with a predetermined transfer function, and particularly to a device and method for selectively transforming an optical wavefront using principles of diffractive optics.
Optical transform devices of various types are commonly employed in optical systems. They work by modifying the wavefront of incoming light in a predetermined way to produce a new wavefront. In mathematical terms, this may usually be thought of as a mapping of one space by which the incoming wavefront is described to another space by which the new wavefront is described. One common optical transform device is a lens with two convex spherical surfaces. Such a lens can be used to focus an incoming wavefront to a point. Mathematically, it performs a Fourier transform on the incoming wavefront. Two other common optical transform devices are mirrors, which can be used to deflect or focus a wavefront, and prisms, which can also be used to deflect wavefronts. Spherical-surface lenses, mirrors and prisms all work according to principles of refraction, and mirrors work according to the principles of reflection. However, there are also optical transform devices that work on principles of diffraction, such as diffraction gratings, which deflect a wavefront, and zone plates, which focus a wavefront.
It is often desirable to employ variable transform devices in practical applications. For example, mechanically-adjustable, multi-element lens systems are commonly used in cameras to provide variable focal length and magnification. One such system is shown in Betensky U.S. Pat. No. 4,466,708 entitled ZOOM LENS. As another example, rotating or oscillating mirrors are commonly used in optical scanners to deflect a beam of light to a selected position. However, such devices have numerous drawbacks. They either must be manually adjusted or driven by a relatively bulky, often heavy and power-consuming motor. They often require expensive precision machining and are subject to mechanical misalignment and wear. Also, practical manufacturing techniques require making compromises in the surfaces of the devices that produce undesirable aberrations in the wavefront. Consequently, numerous efforts have been made to develop electrically-controllable variable optical transforms that do not require a motor.
One such device is a variable focal length refraction lens disclosed by Lotspeich, U.S. Pat. No. 3,424,513 entitled ELECTRO-OPTIC VARIABLE FOCAL LENGTH LENS. It employs a liquid Kerr effect medium which is bulky, difficult to control and requires very high voltage, on the order of 20,000 volts. It also performs a limited type of transform, namely a bi-convex spherical lens transform. Consequently, it is impractical for the majority of optical transform applications.
Max U.S. Pat. No. 3,312,519, entitled WIDE RANGE HIGH SPEED ADJUSTABLE FOCUSING OF HIGH FREQUENCY ELECTROMAGNETIC RADIATION, discloses a type of adjustable zone plate which employs a diffraction pattern of concentric circles produced by an acoustical standing wave which propagates across a parabolic plexiglass surface. It is dependent on and subject to the limitations of an intermediate, electro-acoustic transducer and performs only a limited type of transform, namely a zone plate transform. Consequently, it has relatively limited application.
Another electrically-controllable, variable optical transform is disclosed by Zito U.S. Pat. No. 3,040,635 entitled BEAM SCANNING APPARATUS. In this case, a voltage is applied to prism-shaped cells containing either a variable dielectric material or a Kerr effect material to vary the index of refraction of the cell material and thereby cause the light travelling through the cell to be deflected a selected amount. Like the device disclosed in Lotspeich U.S. Pat. No. 3,424,513, this has the disadvantages of employing a liquid medium that is hard to control, bulky, requires relatively high voltage to operate and performs a limited transform function.
Huiginard et al. U.S. Pat. No. 4,124,273, entitled VARIABLE VERGENCY FOCUSSING APPARATUS, discloses an electrically variable focal length lens system that employs solid Kerr effect cell as a lens. However, as is characteristic of Kerr effect devices, it requires high voltage, on the order of 4,000 volts, to operate and, even then, can only vary the focal point by about 80 micrometers. Also, the index of refraction profile of each lens is dependent on the distribution of electric field between two parallel, relatively thin electrodes, which only approximates a spherical surface. Thence, this device also has limited application.
Several electrically-adjustable optical transform devices using liquid crystals have previously been devised. Liquid crystal material is electro-optically anisotropic such that its index of refraction along one axis can be varied with respect to another, orthoganal axis by the application of an electric potential across the material. It has the advantage that significant changes in the index of refraction along one axis can be effected by a relatively low voltage. For example, a change in index of refraction Δn of about 0.2 can be effected by the application of just a few volts of electric potential across the material.
Bricot et al. U.S. Pat. No. 4,037,929, entitled OPTICAL PROJECTION DEVICE AND AN OPTICAL READER INCORPORATING THIS DEVICE, discloses a hollow convex lens containing nematic liquid crystal material and having transparent electrodes disposed on the opposite interior surfaces of the lens. By applying a selected voltage across the two electrodes, the index of refraction of the liquid crystal, and therefore the index of refraction of the lens, for light of a given polarization may be varied. The focal length of the Bricot et al device can be varied about 15 micrometers, but it requires the construction of a hollow, plano-convex lens and has limited application as an optical transform. A similar device is disclosed in Berreman U.S. Pat. No. 4,190,330 entitled VARIABLE FOCUS LIQUID CRYSTAL LENS SYSTEM, the main difference being that Berreman discloses the use of two back-to-back plano-convex hollow lenses for achieving polarization insensitivity. Kowel et al. U.S. Pat. No. 4,572,616, entitled ADAPTIVE LIQUID CRYSTAL LENS, also discloses a hollow cell having parallel faces and filled with liquid crystal material between transparent electrodes for varying the index of refraction of the liquid crystal material. In this case, at least one side of the cell employs an array of electrodes. The voltages applied to the electrodes are controlled so that the index of refraction across the cell varies smoothly in accordance with the desired profile. One problem with each of these liquid crystal lens devices is that they are based on optical refraction principles. This means that in order to accomplish a substantial variation in focal length the cell must be relatively thick. Thick cells without aberrations due to variations in the surface contour of the cell wall are difficult to construct, and the response time of a liquid crystal cell increases with the thickness of the material.
Liquid crystal material has also been used in a diffractive optical transform device, namely an adjustable diffraction grating, as disclosed in d' Auria et al. French Pat. No. 73 42147. However, the device disclosed in that patent is limited in that it produces multiple diffraction orders, thereby rendering it relatively inefficient as a light beam steering device applicable only to those situations where multiple diffraction orders can be tolerated.
There are a number of applications for which an effective, versatile, electrically-controllable optical transform device could be used. In addition to an adaptive lens and a scanner, as discussed above, mapping one space to another can be employed to switch one set of optical input signals to a selected set of outputs. While liquid crystal material has been employed for this purpose, as disclosed in McMahon et al. U.S. Pat. No. 4,278,327 entitled LIQUID CRYSTAL MATRICES, many configurations used for this purpose experience increased power loss with each additional port and are difficult to construct so as to be polarization insensitive.
Therefore, it would be desirable to have an optical transform device that does not have the problems or limitations of known electrically-controllable optical lens, scanner and switch devices.